1. Field of the Invention
This invention relates to a direct current power supply or “transmutor” for use in mission critical applications, and in particular to an apparatus and method for converting direct current (DC) or alternating current (AC) power into regulated DC power having the same or different voltage levels. The invention employs multiple power circuits and a combination of “droop” compensation and “virtual bus” current sharing (also known as “active current sharing” or “current averaging”) to provide optimal efficiency and load management over an extended load range.
In particular, the invention relates to a DC power supply apparatus and method of supplying DC power for mission critical applications that includes one or more of the following features:                multiple power circuits in one unit, the power circuits being optimizable for efficiency as the load increases or decreases;        A multiphase topology within the power circuits, with logic phase shifts between multiphase;        Two types of power management circuits arranged in parallel or a controller having corresponding programming, or any combination of a programmed controller or components and dedicated circuitry, for implementing: (a) a variable linear or variable exponential precision droop algorithm circuit, and (b) a “virtual paralleling bus algorithm” or current averaging/active current sharing circuit;        Circuitry and/or programmed components for implementing two types of arc detection and suppression, including: (a) unequal positive and negative current in each power circuit to shut down the output power, and (b) monitoring output current for an arc “signature” to protect against arcs between positive and negative.        
2. Description of Related Art
Conventional methods used in transmuting power from one configuration and/or voltage level to another have a single phase internal topology and one circuit to deliver power over the entire load range. However, as society has become more reliant on digital computing systems, it have become necessary to provide ever more reliable sources of low voltage DC power. To accomplish this, it has become common to provide parallel regulated power supplies connected to share the load so that the individual power supplies can operate closer to peak efficiency while still leaving room for increases in power due to increases in the load, or replacement of one or more of the individual power supplies. A problem with such arrangements is that the currents drawn by the loads may vary widely and in different ways, particularly when multiple loads share the power supply, making it difficult to supply a constant output voltage and obtain optimum efficiency.
In general, present multiple power circuit designs have efficiencies of 89 to 93% over a load range of 75 to 110% and are 80 to 85% efficient at ¼ loads. Mission critical loads have N+N redundant power sources that load share and the load power draw may vary by 2:1 depending on the amount of computing the processors are performing. A server, for example, may typically use only ½ rated power with short excursions using 100% power. With N+N redundant power sources sharing the load, each power source would supply ½ the load. Therefore, the power sources normally operate at 50% rated power with short excursions using 100% power. In order to reduce the heat load in a mission critical site, the power sources should have efficiencies of at least 95% from 30% load to 100% load.
The cheapest and easiest solution to high efficiency is to use multiple circuits and load management. For example, in a single power source module topology using four power circuits, each circuit would be rated slightly over ⅖ of power source module rating. Each of the four power circuits would be optimized to have high efficiency from 75 to 110%. As the load is increases, the power circuits are paralleled. The method would attain 95% over the extended range of the load. Any reasonable number of power circuits could operate as described above.
There are currently two principal ways of managing the load in such a system. The first, called the droop method or algorithm, is to simply reduce the output voltage of each individual power circuit y an appropriate amount whenever the current increases. The second, which may also be referred to as the current averaging method, which may be referred to as “virtual bus” method or algorithm, “current averaging,” or “active current sharing,” involves comparison of the output of each individual power source with an average of the total output of the power sources, and generating an error signal to dampen deviations from average by driving the correction signal to zero within the accuracy of the controller.
Numerous ways of achieving droop compensation are known those skilled in the art. For example, compensation for droop can be accomplished by providing a series impedance at the output of the power source. However, to avoid power losses, the same effect can be obtained by monitoring the output current and controlling the power supply output accordingly, for example by changing the duty cycle of multiphase switching signals in a multiphase power circuit, or by varying inductances in the power conversion circuit. In general, more complex droop algorithms can be implemented through the use of digital monitoring and control, including use of non-linear droop algorithms.
There are also numerous ways to implement the virtual bus or current averaging/active current sharing method, including the use of op amps and/or comparators to generate the correction signal by comparing the output of individual power supply with the output or outputs of other power supplies. In addition, virtual bus current sharing can also involve a purely digital control based on inputs from each of the individual power supplies.
Each of these methods has advantages and disadvantages that make one or the other better for particular applications, but not optimal for any particular application. For example, while the droop method provides an immediate response to changes in output of an individual power supply, it is limited by the accuracy of current sensing and cannot adequately account for drift or deviations in the output of the individual current supplies. In general, droop compensation is effective at greater than two or three percent of full load. On the other hand, virtual bus current sharing can provide greater accuracy, but is subject to instabilities in the feedback loop application of the correction factor to an individual power supply will affect the overall current average, causing the correction factor to change, and so forth, necessitating a relatively low bandwidth and making it difficult to keep up with large current oscillations or failure of one or more of the parallel power supplies.
It has previously been proposed to solve these problems by modifying either the droop control or active current sharing/current averaging control methods and apparatus to obtain a hybrid control, as disclosed for example in U.S. Pat. No. 6,201,723 and U.S. Patent Publication No. 2006/0209580. However, the previously proposed methods and apparatus are either overly complicated or fail to achieve desired current compensation over a range of loads/conditions.
In addition, the prior methods and apparatus fail to take into account problems such as arc suppression/prevention that are essential in mission critical applications where failure of the power supply would have severe consequences. Arc faults are more serious in DC power than in AC power since an AC voltage has zero amplitude twice a cycle whereas DC is continuous and the arc is not extinguish until the arc burns a path long enough that the applied voltage will not maintain the arc. The heat from the arc can burn personnel or equipment near by.